In the High Speed Downlink Packet Access (HSDPA) of a third generation (3G) cellular system for Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes, data in the form of Protocol Data Units (PDUs) for the High Speed Downlink Shared Channel (HS-DSCH) is distributed (i.e., buffered and scheduled) in the Node B. Therefore, the Radio Network Controller (RNC) does not have an up-to-date status of the transmissions of Protocol Data Units (PDU).
There are scenarios in which a User Equipment (UE) has to perform a serving HS-DSCH cell change to achieve improved radio conditions and avoid loss of the radio link. The serving HS-DSCH cell change is when the UE has to change the cell associated with the UTRAN access point performing transmission and reception of the serving HS-DSCH radio link.
The Node B associated with the cell before the serving HS-DSCH cell change is called the source Node B and the Node B associated with the cell after the serving HS-DSCH cell change is called the target Node B. With HSDPA, since data is typically distributed in a Node B prior to transmission to the UE, when the UE performs a serving HS-DSCH cell change it is possible that the UE stops transmission and reception in the source cell before all of the PDUs currently stored in the source Node B are transmitted. Accordingly, there is a possibility that considerable amounts of data buffered in the source Node B will be lost. The reason is at the moment of handover there is no mechanism within the UTRAN architecture that allows for transfer of the buffered data to the target Node B. When data is lost in the source Node B it can be recovered by the RNC, but at the cost of significant additional transmission latency that may result in inability to achieve the user's quality of service requirement.
A prior art method for processing data during a serving HS-DSCH cell change is shown in FIG. 1. After the RNC recognizes the need for a serving HS-DSCH cell change, it sends a reconfiguration message to the Node B. This reconfiguration message may or may not specify an activation time, which is an explicit moment in time that is known in the Node B when the UE will stop listening to the HS-DSCH in that cell and start receiving the HS-DSCH in a new cell. If there is no activation time specified in the reconfiguration message, the UE will stop listening to the HS-DSCH in the source cell and wait for receiving the HS-DSCH in a new cell until the Layer 1 connection to the new cell is established. Any data that is buffered in the Node B after the activation time will be stalled in the Node B and is useless and therefore will be discarded.
Upon receipt of the reconfiguration message, the Node B continues to schedule data to UEs based upon the priority of the data and latency requirements. The Node B then applies the appropriate modulation and coding set (MCS), which is chosen by the scheduler, to the data for transmission to the UEs. In current 3G systems, the MCS level is based upon UE feedback that identifies the downlink channel quality to the Node B. Upon reception of the channel quality estimate, the Node B determines the MCS primarily based on a mapping table predefined and known by both the UE and the Node B. The mechanism to choose the MCS may, for example, be based on reaching certain channel quality thresholds. MCS choices range from less robust combinations that provide a high data rate with less error protection, to more robust MCS choices that provide greater probability of successful transmission at lower data rates. The less robust MCS choices use less radio resources for a given data transmission then are required for the more robust MCS choices.
Using the prior art method shown in the flow diagram of FIG. 1, once the activation time expires, the UE is no longer receiving in the source cell and data buffered in the source Node B for transmission in that cell is lost.
The prior art method of recovery of data lost in the source Node B is by radio link control (RLC) layer. The difficulty with the prior art RLC recovery process is that transmission latency is significantly increased and the quality of service requirements may not be achieved. If the number of PDUs stalled in the source Node B is large, the RLC will need to retransmit a large amount of PDUs, resulting in a longer latency of PDU transmission. The transmission delay may be increased further by any new data that is transmitted in the target cell prior to the lost PDUs in the source Node B are known to the sending RLC, since the Node B for each priority queue schedules transmissions as a FIFO regardless of whether the PDUs are initial transmissions or retransmissions. As a result, upon a serving HS-DSCH cell change when data remains buffered in the source Node B, PDUs stalled in the source Node B can result in significant transmission latency for those PDUs.
It is therefore desirable to reduce and potentially eliminate the amount of data that is stalled in a source Node B upon a serving HS-DSCH cell change.